SIMPLE MICROSCOPICAL LENSES. 



29 



behind it than the rays b b, which pass through near 

 its centre or axis, and are less refracted. The conse- 

 quence of this defect of lenses with spherical curva- 

 tures, which is called spherical aberration, is that a 

 well-defined image or picture is not formed by them, 

 for when the object is focussed, for the circumferential 

 rays, the picture projected to the eye is rendered indis- 



Fio. 14. 



tinct by a halo or confusion produced by the central 

 rays falling in a circle of dissipation, before they have 

 come to a focus. On the other hand, when placed in 

 the focus of the central rays, the picture formed by 

 them is rendered indistinct by the halo produced by 

 the circumferential rays, which have already come to 

 a focus and crossed, and now fall in a state of diver- 

 gence, forming a circle of dissipation. The grosser 

 defects of spherical aberration are corrected by 

 cutting off the passage of the rays a a, through the 

 circumferences of the lens, by means of a stop dia- 

 phragm, so that the central rays, b b, only are con- 

 cerned in the formation of the picture. This defect is 

 reduced to a minimum, by using the meniscus form of 

 lens, which is the segment of an ellipsoid instead of a 

 sphere. 



The ellipse and the hyperbola are forms of lenses in 

 which the curvature diminishes from the central ray, 

 or axis, to the circumference b ; and mathematicians 

 have shown that spherical aberration may be practi- 

 cally got rid of by employing lenses whose sections are 

 ellipses or hyperbolas. The remarkable discovery of 



